1. Field of the Invention
The bones of the human body undergo a dramatic change of shape and structure as the body matures until the age of approx 18 years. This is illustrated in FIG. 1, showing a 2-year old child on the left (11) and a 5-year old child on the right (22). This maturation process does not proceed exactly according to the chronological age of the person; it is typical that some children are up to two years ahead or behind.
In medicine it is important to determine the development stage of the skeleton e.g. as an element in the diagnosis of various growth disorders, e.g. growth hormone deficiency, and hand radiographs as shown in FIG. 1 have become the standard means of investigation. The most reliable radiological method used today is the Tanner-Whitehouse (TW) method [1]. In its most common version it utilises 13 bone regions, which are each assigned a stage of maturation manually by visual inspection. There are 8 or 9 stages for each bone described in terms of stylised images and verbal description of radiological features. The TW stages of the radius are shown in FIG. 2, and the children in FIG. 1 thus seem to have radius of stage D (or E) and F respectively. The TW method has defined sex-dependent scores to be attributed to each stage, and the scores thus assigned to the 13 bones are added to form the summed maturity score (SMS). The TW method developers have collected population data of the SMS of normal children, from where they have computed the SMS centiles versus chronological age. This allows the SMS observed for a child under medical examination to be compared with a normal population. Most often the observed SMS is translated into a bone age defined as the age, at which the observed SMS is reached in half of the population.
Another popular scheme is the Greulich-Pyle (GP) method, where the child's X-ray is matched to one of the images in an atlas with images covering all bone ages [12].
There are a number of problems with the TW method: Firstly it is time-consuming. Secondly it is tedious. And thirdly it has a considerable inter-observer variation; the difference between two observers is typically 0.4-0.6 years (SD) or even larger in some instances. Therefore a lot of effort has been invested in automating the procedure using computers, starting in 1989 and culminating in 1992-94, where it was realised that it is technically very difficult, see Niemeijer [2] for a review. The development activity has continued, but still today, no satisfactory automatic system has been made.
2. Description of the Related Art
Thus there is a vast body of prior art in this area and to clarify the new and inventive steps of the invention, the prior art is summarised in the following.
The methods investigated so far, as well as the new invention herein, involve two distinct steps of processing: Bone localisation (sometimes denoted segmentation or reconstruction) and bone interpretation.                A) Bone localisation: The purpose of this step is to identify the bone regions that TW specifies should be rated. Various approaches have been taken in this step:                    a. Manual placement of the region of interest (ROI), by translating, rotating and resizing the image to fit a fixed template            b. Standard pattern recognition methods based on thresholding etc.            c. Snakes            d. Active shape model (ASM)                        B) Bone interpretation: The purpose of this step is to derive the maturity from the bone ROI. Again several methods have been applied                    a. Fourier coefficients            b. Correlation between the image ROI and a template for each stage            c. Width of epiphysis relative to metaphysis            d. Shape parameters (derived from the ASM of the bone localisation)            e. Semantics/rule-based post-processing of features closely emulating the descriptions of TW stages                        The typical idea has been to emulate as well as possible the classification into TW-stages, with the added feature of smoothly interpolations between stages.        